Helmet plenum



FIG. 1 is perspective view of a helmet plenum.

FIG. 2 is a top view of a helmet plenum.

FIG. 3 is a front view of a helmet plenum.

FIG. 4 is a back view of a helmet plenum.

FIG. 5 is a bottom view of a helmet plenum.

FIG. 6 is a side view of a helmet plenum; and,

FIG. 7 is another side view of a helmet plenum. 

CLAIM The ornamental design for a helmet plenum, as shown and described. 